This invention relates to infrared catalytic heaters and, more particularly, to a preheater or starter for an infrared catalytic heater.
Catalytic heaters employ a catalyst bed that results in flame-less combustion of the fuel and the creation of infrared energy. Since combustion is flame-less, these heaters may operate at a temperature that is lower than the ignition temperature of the fuel to the heater, which is typically natural gas or propane. Catalytic heaters are particularly well suited for applications desiring explosion proof operation, such as various applications involving the natural gas industry. In a typical catalytic heater, a catalyst bed is heated to a temperature of about 250xc2x0 F. at which time a thermostat valve is opened so that the supplied fuel and oxygen form the desired reaction with the catalyst bed.
One method used to start the heater is to supply fuel to the back of the catalyst pad while heating a portion of the surface of the catalyst bed with a flame, such as shown in U.S. Pat. No. 5,993,192. After the reaction is established, the flame is removed. The open flame starting method is more dangerous than the electric heater starter and is not acceptable for use in many hazardous locations.
Infrared catalytic heaters normally require the catalyst pad or bed to be heated above the activity temperature of about 250xc2x0 F. to start the catalytic reaction. A resistive electric heating element located between the catalyst pad and the insulation pad is commonly used to preheat the catalyst above the activity temperature. When the catalyst is hot enough, i.e., at or above the activity temperature, fuel enters the back of the heater while oxygen from the air is diffused through the front. When the oxygen and fuel converge in the catalyst, an oxidation reaction takes place resulting in a flame-less combustion and creating infrared energy. After the oxidation reaction begins, power to the electric heating element can be removed and the reaction will continue until either the fuel supply or the oxygen is eliminated.
A 6 inch by 6 inch square heater will typically use a 150 Watt, 0.30 inch diameter, heating element forming a 4 inch diameter circle between the catalyst pad and the insulating pad. Power is applied to the electric starter element for 15 to 20 minutes to preheat the catalyst pad. To start the 6 inch by 6 inch heater requires 37.5 Watt/hours (150 Wattsxc3x970.25 hours=37.5 Wh). If a 12 volt battery is used to power the starter, over 3 Amp/hours is used every time the heater is started (37.5 Wh/12 volts=3.12 h). Larger heaters will use more power because the starting element is larger. For example, a 24 inch by 36 inch heater made by CCI Thermal Technologies Inc. of Alberta, Canada, requires a 1200 Watt starting element. After the 15 minute warmup, propane or natural gas is supplied to the back of the heater at a controlled flow rate and the catalytic reaction begins. After the reaction is fully established, power to the starting heater is removed.
When infrared catalytic heaters are used in hazardous locations, such as gas pipeline equipment, the power source for the electric heating element must be located away from the catalytic heater and in a safe location. Typically the electric connections for the starter heater are made in an explosion-proof housing on the back of the heater and then conductors run through rigid conduit to the safe area which can be 25 feet or more away from the heater. Lowering the maximum output of the starter power source results in a smaller power source, and smaller wire and conduit to connect the power source to the heating element, thereby significantly reducing the installation cost and increasing safety, especially for the high power heaters.
The disadvantages of the prior art are overcome by the present invention, and a catalytic heater with an improved electrically powered starter and method of starting and controlling the catalytic heater as hereinafter disclosed.
In a preferred embodiment, the starter for the infrared catalytic heater utilizes a low cost, low power quartz-halogen lamp to preheat a small section of a catalyst pad to the activity temperature. Several unique features of the halogen lamp make it an efficient starter for the catalyst pad. The lamp envelope is small to reduce the amount of halogen gas required and reduce the cost of the lamp. The small envelope also keeps the tungsten element close to the lamp walls, thereby allowing high heat transfer to the envelope. The heating element is hermetically sealed inside the quartz envelope, thereby eliminating corrosion issues associated with conventional resistive elements.
The starter is preferably inserted into the side of the catalyst pad and spaced closer to the catalyst exterior face than to the front face of the insulation, so that the starter is surrounded by catalyst pad material. Only a small section of the pad need be preheated to the activity temperature for the catalytic reaction. After the reaction begins in the starter area, the heat generated by the exothermic catalytic reaction spreads across the catalyst pad and the startup is complete. Power to the starter is removed after partial catalyst activity takes place to maximize power savings.
The low power requirements of the quartz-halogen starter make it possible to include a small battery pack in an explosion-proof box near the heater for the starter power. This eliminates the long conductor wire and conduit run to the safe area for the starter power source, thereby enabling the heater to be a self-contained unit.
It is an object of this invention to provide a catalytic heater with an improved low power preheater or starter.
A further object of the invention is the provision of such a starter that heats a relatively small portion of the catalyst pad to the desired temperature in a minimum of time. Less than 2% of the flow through area of the pad preferably defines the perimeter area of the heating element, while in the prior art the perimeter area of the heating element was typically 50% or more of the catalyst pad flow through area. The cross-sectional of the heating element itself relative to the flow through area of the pad is also very small, and preferably is less than 2% of the flow through area of the catalytic pad. The wattage for the electrically powered heating element is low relative to the wattage of prior art catalytic starters, and according to the present invention is less than 15 watts regardless of the flow through area for the catalytic pad.